Referring to FIG. 1, a pressure swing adsorption oxygen concentrator O typically uses two sieve bed containers S1, S2 connected at one end to a shuttle valve V and at the other end to an oxygen-enriched air storage tank T. The shuttle valve V, controlled by controller R, is used to alternate flow of air from a compressor C to each sieve bed S1, S2 in turn. As illustrated, air from the compressor C is being provided to the sieve bed S1, and the sieve bed S2 is being vented via line N2 to ambient atmospheric pressure, or line N2 could be connected to the intake of the compressor C to provide a negative pressure field in S2. When the valve V is shifted by controller R, bed S1 is connected to line N1, which vents to atmosphere or is connected to the compressor intake, and bed S2 is connected to compressor C, to pressurize bed S2 and vent bed S1.
In order to effect adsorption of nitrogen molecules from the air, the air flows through the sieve material of the bed being pressurized by the compressor and achieves a pressure over time as the sieve bed S1 or S2 fills with pressurized air. The end of the sieve bed opposite from the compressor C exhausts oxygen-enriched air into the storage tank T that is then connected to a patient at P by way of a device (not shown) that allows the patient to breath the oxygen-enriched air. FIG. 1 is a simplified view of a typical oxygen concentrator, it being understood that an oxygen concentrator may include many other components such as dehumidifiers, other valves, etc.
Once sufficient time at pressure is attained in the sieve beds S1 or S2 being pressurized by the compressor, the shuttle valve V is switched, typically through the action of a solenoid (i.e., it is typically a solenoid valve). This allows the first sieve bed to exhaust to atmosphere (or to the intake side of the pump as explained above) and in the process release the nitrogen molecules that were stored in the sieve as a portion of the oxygen-enriched air from the storage tank flows back through the sieve bed in the reverse direction. While the first sieve bed exhausts and is recharged, the second sieve bed receives air from the compressor and repeats the same process as the first sieve bed. The process is repeated as the shuttle valve again moves back to the first position causing the first sieve bed to be filled again and the second sieve bed to be recharged, and so on.
The compressors C most often used for oxygen concentrators are positive displacement, wobble-piston type reciprocating pumps. As a positive displacement device, the torque required of the motor driving the pump increases quickly during starting as pressure is generated, resulting from the filling of the sieve bed. Permanent split capacitor type motors are most often used partially due to their inherently high starting torque characteristics as compared to shaded-pole motors. If the pressure build-up could be minimized until the motor reached full running speed, the starting torque characteristics of less expensive motors, for example shaded pole motors, would be sufficient to operate the compressor. The resulting cost of the compressor could be significantly less using such motors instead of a higher starting torque motor. Thus, it would be desirable in an oxygen concentrator to relieve the pressure build-up long enough to allow a lower starting torque so as to bring the compressor up to a speed at which a lower starting torque motor can sustain operation without stalling.
Unloading devices are used in conjunction with compressors in systems where the compressor would otherwise be required to start against a back pressure load. Tank-mounted compressors are an example. Typically, a spring-actuated valve that is closed by an electromagnet during operation and opened by a spring is used to bleed off pressure in the line between the compressor head and the tank when the compressor power shuts off. In other systems a flow sensitive valve is used that opens the line from the compressor to the tank once flow is reduced as the compressor comes to a stop. Thus, it would be desirable to eliminate the need for a separate unloading valve of the types described, thereby further reducing the overall cost of an oxygen concentrator.